Projected sensitivities of the LUX-ZEPLIN experiment to new physics via low-energy electron recoils

LUX-ZEPLIN is a dark matter detector expected to obtain world-leading sensitivity to weakly-interacting massive particles interacting via nuclear recoils with a $\sim 7$-tonne xenon target mass. This paper presents sensitivity projections to several low-energy signals of the complementary electron recoil signal type: 1) an effective neutrino magnetic moment, and 2) an effective neutrino millicharge, both for $pp$-chain solar neutrinos, 3) an axion flux generated by the Sun, 4) axionlike particles forming the Galactic dark matter, 5) hidden photons, 6) mirror dark matter, and 7) leptophilic dark matter. World-leading sensitivities are expected in each case, a result of the large 5.6 t 1000 d exposure and low expected rate of electron-recoil backgrounds in the $<100\mathrm{keV}$ energy regime. A consistent signal generation, background model and profile-likelihood analysis framework is used throughout.

Figure 1

The ER backgrounds expected in LZ, after application of veto anticoincidence, single-scatter, and fiducial-volume selections. The left axis indicates rate in standard units; the right axis indicates counts per keV in the anticipated 5.6 tonne 1000 day LZ exposure. This is true energy, where effects of detector resolution and threshold are not included. The three dashed curves indicate the three species that are present as contaminants within the LXe, for which some uncertainty exists on their final concentrations. The curve labeled ‘Det.+Sur.+Env.’ identifies contributions from the Compton scattering of $\gamma$-rays emitted from the bulk and surfaces of detector components, and from the laboratory and rock environment. Three monoenergetic peaks from ${}^{124}\mathrm{Xe}$ double electron capture decay are indicated as lines (for which the units are counts/kg/day and counts on the left and right axes, respectively).

Figure 2

Spatial distribution of all ER backgrounds passing single scatter and veto anti-coincidence selection criteria, with energy less than 100 keV. The 5.6 tonne fiducial region is indicated by the dashed black line.

Figure 3

ER signal spectra studied in this work. True deposited energies are shown using a dotted line and reconstructed energies that include smearing and threshold effects with solid lines. The expected background, also including experimental effects, is shown as the gray region. Each signal spectrum is scaled to the 90% C.L. rejection sensitivity, as described in Secs. 6 and 7. The right vertical axis specifies counts/keV in the 5.6 t 1000 d exposure, and is intended to give the reader a sense of the expected statistical fluctuations.

Figure 4

Projected 90% C.L. exclusion sensitivity to (a) electromagnetic neutrino couplings ${\mu }_{\nu }$ and $q_{\nu }$, (b) kinetic mixing squared, ${\kappa }^2$ for hidden photons, (c) axioelectric coupling for solar axions, (d) axio-electric coupling for galactic ALPs, (e) mirror dark matter kinetic mixing, and (f) DM-electron scattering cross section for leptophilic dark matter. $\pm 1\sigma$ (green) and $+2\sigma$ (yellow) bands are also shown. For selected models, sensitivity to $3\sigma$ evidence is also shown (labeled ‘LZ $3\sigma$ Significance’). Results from other experiments and astrophysical constraints are referenced in the text.

Figure 5

Variation of 90% C.L. exclusion sensitivity to (a) solar neutrino magnetic moment, (b) solar neutrino effective millicharge, (c) kinetic mixing squared, ${\kappa }^2$ for 15 and $70\mathrm{keV}/c^2$ hidden photons, and (d) mirror dark matter kinetic mixing for local mirror electron temperature 0.3 keV. $\pm 1\sigma$ (green) and $+2\sigma$ (yellow) bands are also shown.